In Search of the First Stars: An Ultra-Compact and Very Low Metallicity Lyman-$α$ Emitter Deep Within the Epoch of Reionization

Chris J. Willott, Yoshihisa Asada, Kartheik G. Iyer, Jon Judez, Gregor Rihtarsic, Nicholas S. Martis, Ghassan T. E. Sarrouh, Guillaume Desprez, Anishya Harshan, Lamiya Mowla, Gael Noirot, Giordano Felicioni, Marusa Bradac, Gabe Brammer, Adam Muzzin, Marcin Sawicki, Jacqueline Antwi-Danso, Vladan Markov, Roberta Tripodi

Published 2025-02-11Version 1

We present {\it JWST} observations of a gravitationally-lensed, extremely metal-poor galaxy at redshift $z=8.203\pm 0.001$ from the CANUCS survey. Based on the low oxygen to Balmer line ratios we infer a gas-phase metallicity of $12+{\rm log(O/H)}=6.85$ (1.4\% solar), making CANUCS-A370-z8-LAE the most metal-poor galaxy known at $z>7$. With a high H$\beta$ equivalent width of $225\pm50$\,\AA\ and a half-light radius of only $r_{\rm hl} = 38 ^{+3}_{-19} $\,pc, the galaxy has a high star-formation-rate density of $50 - 100\,M_{\odot}$\,yr$^{-1}$\,kpc$^{-2}$. The galaxy shows high equivalent width Lyman-$\alpha$ emission with an inferred Lyman-$\alpha$ escape fraction of $0.21 \pm 0.05$. The high escape fraction of Lyman-$\alpha$ is likely due to the compact starbursting nature of the galaxy combined with its location in an overdensity traced by at least two other galaxies spectroscopically confirmed to lie within $\delta z = 0.01$ that have helped to reionize the environment. The low metallicity of CANUCS-A370-z8-LAE is best explained by a model where infalling metal-poor gas dilutes the interstellar medium, rather than being a young galaxy forming its first stellar populations.